Field of the invention
The present invention concerns a procedure for the transmission of information, and particularly a procedure for reconstituting information received from samples obtained by sampling using a clock asynchronous with the received information signal. The invention is particularly applicable to reading magnetic records for VCRs, computer peripherals and special professional recorders.
More precisely, the invention does this reception by applying a Maximum Likelihood Sequence Estimation, hereinafter referred to as MLSE, and which uses a VITERBI algorithm described in the document "Principles of Digital Communications and Coding" by A. J. VITERBI et al, McGraw-Hill, 1979. The MLSE algorithm is described in the article "Maximum Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interferences" by G. David FORNEY, IEEE Transactions on Information Theory, vol. IT-18, No 3, pp. 363-378, May 1972. This algorithm has the constraint that a sampled signal must be processed synchronously with the data at a frequency f.sub.e equal to the frequency f.sub.s of the data signal.
In the case of an asynchronous system, preprocessing has to be carried out in addition to equalization to correct the phase using a Phase Locked Loop (PLL) interpolation. These operations do not give good results for withdrawal, in other words for very low frequencies f.sub.e. Therefore f.sub.e is taken&gt;f.sub.s (but as the fractional ratio for hardware installation reasons), such that decimation is subsequently necessary in order to obtain the right frequency.
Therefore, the present invention concerns application of the MLSE algorithm to asynchronous reception and extends to the case of data sampled at a frequency f.sub.e other than the signal frequency f.sub.s. This means that a phase calculation has to be carried out between a data signal and the sampling clock.